Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Investments, Owned, Federal Income Tax Note [Line Items]
Federal statutory rate	21.00%	35.00%
Decrease in tax reserve deductible temporary difference			$ 4,860,000
Offsetting tax reserve deductible temporary difference against taxable income			$ 4,860,000
Tax reserve against taxable income amortization period			8 years
Income tax expense (benefit)	$ 527,000	$ 575,000
Valuation allowance for deferred tax assets	87,716,000		$ 78,697,000
Unrecognized income tax benefit, net	2,637,000		2,637,000	$ 4,391,000
Unrecognized income tax benefit, gross	12,557,000		12,557,000
Operating loss carry forward	$ 543,803,000		554,553,000
Operating loss carry forward, expiration period	15 years
Alternative minimum tax credit carry forward	$ 3,195,000		4,216,000
Penalties expense related to federal income tax	0	0
Interest expense (income) related to federal income tax	(7,000)	37,000
Interest payable arising from income tax examinations	470,000	$ 477,000
Contributions from (distributions to) capital to Transamerica Corporation	15,047,000		(47,465,000)
Foreign
Investments, Owned, Federal Income Tax Note [Line Items]
Tax credit carry forward	$ 9,916,000		$ 9,831,000
Foreign tax credit carry forward, expiration date in year	10 years		10 years
Capital Loss Carry forward
Investments, Owned, Federal Income Tax Note [Line Items]
Tax credit carry forward	$ 0		$ 0
Earliest Tax Year
Investments, Owned, Federal Income Tax Note [Line Items]
Operating loss carry forward, expiration year	2023
Latest Tax Year
Investments, Owned, Federal Income Tax Note [Line Items]
Operating loss carry forward, expiration year	2031
Latest Tax Year | Foreign
Investments, Owned, Federal Income Tax Note [Line Items]
Foreign tax credit carry forward, expiration year	2028